1. Field of the Invention
The present invention is directed to cryopump used in limited space applications.
2. Discussion of the Related Art
Cryopumps are typically made with the inlet in a plane perpendicular to the axis of the expander cylinder, “in line”, or parallel to the axis of the cylinder, “low-profile”. Low profile cryopumps are preferred over in line cryopumps in applications where space is limited and are usually mounted under or on the side of the vacuum chamber.
Two stage G-M (“Gifford-Mchon”) refrigerators are used to cool cryopumps. These cool a first stage cryopanel at 50 to 100 K and a second stage cryopanel at about 15 K. The expander is usually configured as a stepped cylinder with a valve assembly at the warm end of the first stage, a first stage cold station (at 50 to 100 K) at the transition from the larger diameter first stage to the smaller diameter second stage, and a second stage cold station (at about 15 K) at the far end. An example of an expander that is used in cryopumps as described in this application is found in U.S. Pat. No. 6,256,997.
U.S. Pat. No. 4,150,549 describes a typical in line cryopump that uses a two stage G-M refrigerator to cool two axi-symetric cryopanels. The first stage cools an inlet (warm) panel that pumps Group I gases, e.g. H2O and CO2, and blocks a significant amount of radiation from reaching the second stage (cold) panel but allows Group II gases, e.g. Ar and N2, and Group III gases, e.g. H2 and He, to pass through it. The Group II gases freeze on the front side of a cup shaped cold panel and Group III gases are adsorbed in an adsorbent on the backside of the cold panel. U.S. Pat. No. 4,530,213 describes a cold panel design that consists of a series of concentric rings of increasing diameter from the inlet region to the back of the housing. This design has more room for large amounts of Ar to collect than the cup design, as is the case with sputtering, and there is more surface area on which the Ar is distributed.
U.S. Pat. No. 4,530,213 shows an arrangement that is typical for in line cryopumps of having heat from the inlet louver, typically consisting of segments of circular cones, transported radially to the inlet end of the warm panel then conducted to the first stage heat station through the warm panel. A similar arrangement is shown for a low profile cryopump in U.S. Pat. No. 5,974,809. U.S. Pat. No. 6,155,059 shows an in line cryopump with straight inlet louvers and two straight bars, thermal busses, that carry heat to opposite sides of the inlet end of the warm panel.
U.S. Pat. No. 4,691,534 describes straight louvers that are individually attached to the warm panel at the inlet end. U.S. Pat. No. 5,542,257 shows in FIGS. 1 and 2 a low profile cryopump that has straight inlet louvers and a single cross bar that transfers heat radially to opposite sides of the warm panel inlet.
U.S. Pat. No. 5,056,319 describes a low profile cryopump with a vibration isolation mechanism. The drawings show straight inlet louvers with a cross rod taking heat to the warm panel but the physical description is lacking.
U.S. Pat. No. 4,356,701 describes an in line cryopump that has conventional conical inlet louvers with radial bars to conduct heat but these bars are not connected to the inlet end of the warm panel, rather they are connected to rods that conduct heat through the second stage cryopanels to the bottom of the warm panel.
U.S. Pat. No. 5,156,007 and U.S. Pat. No. 5,974,809 show a shield that has to be added over the second stage cylinder to avoid having Ar or N2 freeze at some temperature above the cryopanel temperature. The phenomena of “Ar hang up” that results from Ar freezing on the second stage cylinder is described in U.S. Pat. No. 5,156,007.
The cryopanels for in line cryopumps are typically axi-symetric around the expander cylinder. This panel design is frequently adapted to low profile cryopumps by having cutouts in the cold panel for the expander cylinder, e.g. U.S. Pat. No. 5,156,007. This invention improves upon previous designs by having second stage cryopanels that are comprised of flat surfaces that form a nested tent like structure that is attached to the second stage heat station and extends over the second stage cylinder, between the cylinder and inlet, to shield the cylinder from having Group II gases freeze on it. The second stage heat station does not have to be in the middle of the housing because the second stage cryopanels, which consist of folded flat sheets of copper, can be attached any place along their length.